Re: Concurrent MERGE

robertmhaas(at)gmail(dot)com (Robert Haas) writes:
> On Thu, Aug 5, 2010 at 11:43 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> Looks like MERGE is progressing well.
>>
>> At 2010 Dev Mtg, we put me down to work on making merge work
>> concurrently. That was garbled slightly and had me down as working on
>> predicate locking which is the general solution to the problem.
>>
>> Do we still need me to work on concurrent MERGE, or is that included in
>> the current MERGE patch (can't see it), or is that covered elsewhere
>> (for example Kevin Grittner's recent work)?
>>
>> Still happy to do work as proposed, just checking still required.
>
> I suspect Kevin's patch will solve it if using a sufficiently high
> transaction isolation level, but something else might be needed
> otherwise. However, I confess to ignorance as to the underlying
> issues? Why is MERGE worse in this regard than, say, UPDATE?

It's worse than UPDATE because
- It could be an INSERT, if the data's new, but
- If the data's there, it becomes an UPDATE, but
- If some concurrent update has just DELETEd the data that's there, it
becomes an INSERT again, but
- Oops, that DELETE rolled bac, so it's an UPDATE again...

Recurse as needed to make it more undecidable as to whether it's really
an INSERT or an UPDATE :-).
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